Abstract
This paper presents an in-depth scientific exploration into the coupled aerodynamic and structural characteristics of a flexible flapping wing undergoing passive elastic twist along with bending. The purpose is to fundamentally understand the aeromechanics with regards to the generation of lift and its relationship to wing deflection for an aeroelastically tuned flapping wing used on a bio-mimetic robotic hummingbird capable of sustained and controlled hovering flight. A specially designed experimental rig was constructed in which a miniature beam force sensor was placed at the wing root to measure the instantaneous aerodynamic and inertial vertical loads experienced by the wing during flapping. In order to separate out the inertial loads from the aerodynamic loads for a flexible wing, a novel experimental procedure was developed and implemented in which the displacement of the flexible flapping wing was measured accurately at high spatial and temporal resolution during flapping using digital image correlation (DIC) techniques, which was used to obtain the acceleration vector at every surface element on the wing; the surface acceleration multiplied with the mass of the element provided the instantaneous inertial force at each surface element of the wing, which was subtracted from the total measured force to obtain just the instantaneous aerodynamic vertical force or lift. Additionally, using these deflection measurements, it was found that the wing had an approximately linear twist rate, and that the larger the twist rate, the higher the lift. A maximum camber of 10% along most of the wing was also observed. Finally, the flow-field and key aerodynamic structures of the flexible flapping wing were resolved using chord-wise and span-wise particle image velocimetry (PIV) measurements which showed attached flow at steep angles of attack during the acceleration phase of the wing, and the development and bursting of a strong leading edge vortex with a diameter equal to the wing chord at the 70% span location during deceleration. This aeromechanical analysis marks a key development in the characterization of hummingbird-like flapping wings used on a hover-capable robotic hummingbird, and the design of flexible, structurally-tuned wings for micro air vehicle applications.